Transmit power control plays an important role in interference-limited communication networks, such as those based on Code Division Multiple Access (CDMA) technologies. Reliable communication and targeted levels of data throughput require transmission at sufficient power to insure adequate received signal quality, but transmitting at excess power is avoided as a mechanism to limit or otherwise reduce interference.
As one example of transmit power control, when a mobile terminal transmits an information signal over a communication channel to a base station, the base station feeds back transmit power control (TPC) commands to the mobile terminal. These TPC commands direct the mobile terminal to increase, decrease, or maintain the transmit power of the communication channel, as needed to keep the received signal quality at the base station at a targeted level over changing fading conditions.
Some contexts complicate the above approach to transmit power control. For example, in Dual-Cell High-Speed Uplink Packet Access (DC-HSUPA) systems, a mobile terminal may simultaneously transmit two different communication channels on two different carriers (i.e., in two different frequency regions). Transmitted in different frequency regions, the channels may experience different fading conditions. Accordingly, the channels are independently power controlled, meaning that the mobile terminal receives separate TPC commands for them. As a result, however, the transmit power of one channel can become much greater than the transmit power of the other channel (e.g., in the case of soft handover between different base stations). When this happens in practice, non-linearities in power amplification and/or IQ imbalances in modulation cause the greater powered channel to leak into and interfere with the lesser powered channel. Such self-interference degrades the quality of the lesser powered channel. (See, for example, FIGS. 1A and 1B, which illustrate degradation of the lesser powered channel transmitted at frequency f2 due to interference from the greater powered channel transmitted at frequency f1).
A different, yet somewhat related complication arises in Long Term Evolution (LTE) systems, where a mobile terminal can simultaneously transmit the Physical Uplink Control Channel (PUCCH) and the Physical Uplink Shared Channel (PUSCH) in two different frequency regions. Transmitted in different frequency regions, the PUCCH and the PUSCH are independently power controlled in much the same way as described above. Here, though, complications arise mainly when the power of one channel is approximately the same as the power of the other channel. At relatively high power levels, intermodulation products may introduce significant spectral peaks outside of the two frequency regions allocated for the transmission. These unwanted spectral emissions may substantially interfere with other communication systems.